1. Field of the Invention
The field of the present invention is deformable mirrors.
2. Background
Deformable mirrors are mirrors whose reflective surfaces can be deformed in a manner that allows for control of the wavefront of the reflected light. The need for deformable mirrors arises in adaptive optics for wavefront control and correction of optical aberrations. Deformable mirrors are often used in combination with wavefront sensors in real-time control systems. In such systems, the wavefront sensor communicates the measured wavefront to a computer which determines how the deformable mirror surface should be shaped in order to achieve the desired wavefront. Mechanical or acoustic actuators control the surface shape of the deformable mirror and receive commands from a controller coupled to the computer to shape the surface of the deformable mirror accordingly.
There are two main types of deformable mirrors, continuous faceplate and segmented. Continuous faceplate deformable mirrors have discrete actuators coupled to the back surface of a thin deformable faceplate, and the actuators largely control the surface shape of the faceplate. The overall surface shape of the plate depends on the combination of forces applied to the faceplate, including forces from individual actuators, combinations of actuators, boundary conditions, and the geometry and the material of the plate. Continuous faceplate mirrors are generally preferred over segmented deformable mirrors, since the former allow smooth wavefront control with very large-up to several thousands-degrees of freedom.
Segmented deformable mirrors are formed by independent mirror segments. Each segment may be moved up or down freely with no inter-segment coupling. This type of movement results in a discontinuous surface that is a stepwise approximation of the desired wavefront, and such surfaces work poorly for smooth, continuous wavefronts. Sharp edges of the segments and gaps between the segments contribute to light scattering and heating between the segments. Both of these undesirable effects are amplified when a segmented deformable mirror is used with higher power light sources, thus limiting the applications for such mirrors.
The deformable mirrors commonly used in conjunction with high energy laser (HEL) systems are of the continuous variety. HEL systems generally involve the use of a laser or other source of a high-power directed electromagnetic energy for any one of a number of purposes. During use, the laser heats and can create distortions within the optical system. While a deformable mirror is designed to correct the internal optical distortions, it too is heated and distorted in ways it cannot correct, leading to degradations in HEL performance.
Current high energy level deformable mirrors have a fairly common architecture, consisting of actuators, which are often piezo-stacks, mounted on a base plate and attached to a thin facesheet through metal flexures. Each actuator is adjusted in piston to create an overall deformation of the facesheet, which in turn alters the wavefront of the reflected HEL beam. With this arrangement, the thin facesheet minimizes the bending stiffness of the mirror in response to the pistoning actuators. One shortcoming of this type of deformable mirror design is that it is subject to significant thermal deformation, which causes, optical distortions, when used with high powered lasers or with lasers of lesser power for extended periods of time. The thermal deformation arises because the thin facesheet is heated by the laser and heats up tremendously as compared to the actuators and base plate, resulting in a differential in thermal expansion which twists the flexure attachments to the actuators, thereby causing ripples in the surface of the facesheet. Also, the facesheet and each of the flexure attachments can have a local mismatch in thermal expansion, thereby causing a local curvature above each actuator and a dimple effect across the surface of the facesheet.
With continuous deformable mirrors, there are two potential approaches for reducing optical distortions caused by thermal deformation of the facesheet. One option is to use a thicker facesheet. However, use of a thicker facesheet reduces the flexibility of the deformable mirror surface, and hence reduces the wavefront correction capability. Another option is to use a segmented mirror. However, as discussed above, segmented mirrors have their own shortcomings when used with an HEL beam.